The finger joint is a common joining technique used in woodworking to join two boards together, often at right angles. Its most common application is in the making of boxes. For this reason, the finger joint is sometimes referred to as a "box joint."
In a finger joint, the ends of both boards are cut to form a series of fingers separated by spaces, or "kerfs." The boards are joined by interlocking the fingers of one board with the fingers of the other board and using an adhesive to maintain the joint. The key to a good finger joint is that the width of the fingers and the width of the kerfs be identical, providing a tight fit when the fingers are inserted into the kerfs.
Outside of a production shop, finger joints are most often cut using either a router or a table saw. The router approach typically uses a template to guide the router. The template will have a separate slot for each kerf to be cut. the width of the finger is controlled by the spacing of the template and the width of the kerf is dictated by the width of the router bit. The table saw approach typically uses a fixture, or jig, to hold the board in position, on end, as it is passed over the table saw blade. After each pass the board is moved to a new position in the jig, often aligning the most recently cut kerf with a key to align the board for the next cut. The width of the fingers is controlled by the distance between the key and the blade and the width of the kerf is determined by the thickness of the blade. Dado blades are often used to provide wider cuts.
The traditional approach to using a jig to cut finger joints on a table is described in Finger Joints, Woodsmith, v. 19, #110, April 1997 (Woodsmith). The starting point for the jig is a fence, perpendicular to both the table and the blade which is fixed in position laterally relative to the blade but free to slide in the direction of the cut. This is usually accomplished by fixing a board to the table saw's miter gauge and setting the gauge for a 90 degree cut. The fence may be supplemented with a ledge, attached to the bottom of the fence, parallel to the table. The ledge supports the board being cut and provides support for the key. With the fence in place, the steps for setting up the jig are: 1) adjust the blade to the thickness of the stock; 2) cut a notch in the fence, and ledge; 3) make a key, sized to fit tightly in the notch and glue it in place; 4) using a spacer the same size as the key, adjust the fence so that the space between the blade and the key is the width of the spacer (and thus the key); 5) test the jig and make adjustments to the key location and blade height until a tight joint is achieved; 6) fix the fence in position with screws and/or glue.
In the above process, steps 3 and 5 are critical to the setup of the jig and can be very time consuming. The key used in step 3 is quite small, often one to one and one half inches long and the thickness of the blade being used, often one eighth to one quarter inch. Because of this, it must be shaped by hand and must be an exact match for the width of the blade. Inaccuracies in the key may result in a loose fit in the notch and make the adjustments in step 5 more difficult.
Step 5 is the final adjustment of the jig. A test joint is made by cutting two boards and fitting them together. If the joint is loose, the key (and fence) must be adjusted to move away from the blade. If the joint is tight, the key must be moved toward the blade. After the adjustment, a new test joint is cut, and the process is repeated. The adjustments in the key position may be in increments of 1/64ths of an inch (or less) and several iterations may be required before a tight joint is achieved.
Finger joint jigs are also available commercially. One example is the "Aluminum Box Joint Jig" available from Woodsmith Shop (Summer 1995 catalog, pg. 18). This jig provides a fence and ledge as described above but also provides a built-in adjustable key to alleviate the problems associated with step 3 above. It also provides a threaded adjuster for setting the gap between the key and the blade. However, the basic approach is as described above, and iteration is still required in step 5 to obtain a good joint.
Once the jig is properly set, a large number of high quality joints can be cut it relatively short time. However, the trial and error approach to setting the jig can result in a high level of frustration for the woodworker. Where a single box is being made, the set-up time can easily exceed the time needed to actually cut the joint. Because of this, the finger joint may be avoided by amateur woodworkers and even professionals who work in very small quantity.
There is need for a finger joint jig which eliminates the trial and error setup described above and which preferably does not require the hand fitting of the key piece to the slot cut by the blade.